Magnetic resonance imaging (MRI), the most widely used imaging modality for soft tissues, in general lacks sufficient sensitivity for molecular imaging of biological processes associated with cancer. Although Gd3+based contrast agents are widely used in clinical MRI as non-specific extracellular agents, new approaches need to be developed to bring MRI into competition with optical and nuclear methods for molecular imaging of cancer. A new class of responsive paramagnetic chemical exchange saturation transfer (PARACEST) agent has been designed as highly specific reporters of tumor physiology and metabolism. Although several groups have recognized their potential importance as molecular imaging agents, their practical application to tumor imaging has not been realized at this point largely because of background interference from the inherent magnetization transfer (MT) signal from all tissues. In this project, we propose a practical solution to eliminate this background MT signal so the inherent molecular specificity of these reporter molecules can be fully realized in vivo. A novel molecular design is proposed that will yield a platform of responsive MRI agents for discriminating regions of low pH, hypoxia and reactive oxygen species (ROS) in tumors using standard clinical MRI scanners. These three indices, pH, hypoxia and excess ROS, are all hallmarks of rapidly proliferating cells so having a stable of responsive agents capable of sensing these tissue biomarkers would be extremely valuable for monitoring tumor metabolism. Our objective is to implement all three responsive agents in vivo in animal tumors using agent concentrations that would be considered acceptable for human use and without interference from tissue MT signals by the end of this 5 year project. A second newer class of probes called T2exch agents will be tested and evaluated as reporters of cancer-related protease activities by MRI. If successful, the agents developed in this project will make MRI competitive as an effective molecular imaging tool for cancer diagnosis.